, issued October 24, 1961, proposes a design for a "Toy building brick." Today, you may know these blocks as LEGOs, produced by the billion-dollar company that has cornered the market on savvy themed sets.
The description for the patent cites a Danish approval for the same design in 1958. That was already over 25 years after LEGO had been founded, but this particular patent, as the document states, introduced the concept of
… building bricks or blocks adapted to be connected together by means of projections extending from the faces of the elements and arranged so as to engage protruding portions of an adjacent element when two such elements are assembled ... thus providing for a vast variety of combinations of the bricks for making toy structures of many different kinds and shapes.
The patent has since expired—opening the market to competitors—but LEGO's brilliant interlocking design has kept them well ahead of newer knock-offs.
[h/t Design Is Fine]